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New research from Bankrate.com finds that one in four Americans owes more in debt than they have in emergency savings. Another survey, this one from the Consumer Federation of America and the American Savings Council, found that nearly half of us believe we aren’t saving enough to provide a desirable standard of living in retirement.

Factor in the accelerating decline of Social Security. and it’s easy to understand the allure of brokerage credit cards.

Never heard of those? Think cash-back cards, but the cash back is added automatically to your investment account, where it grows like the rest of your portfolio.

Shop, earn cash rebates, and then deposit the kickbacks into a brokerage account for capital appreciation. Free money creating more free money: What could possibly go wrong?

Plenty. Just like any other type of card, brokerage cards charge interest on balances. They’re also more likely to charge annual or transactions fees because of the benefits of depositing directly into any of a multitude of accounts.

Bigger Portfolio or Bigger Fees?

Last year, broker Charles Schwab (SCHW ) stopped promoting its card for account holders, shunting existing cardholders to an offering from Bank of America (BAC ). Yet some of Bank of America’s benefits can actually result in higher fees if you’re not careful. For instance, with the BankAmericard Privileges account, you get 50% more cash back if you deposit into a B of A checking account. Sounds great, except that you’ll pay account maintenance fees (at minimum) unless you arrange for regular direct deposits or keep a minimum balance. Nationally, Bank of America has been testing accounts that charge $6 to $25 monthly, according to The Charlotte Observer .

Fortunately, you needn’t pay fees to get a good return on your shopping. The trick is to know the limitations when buying your way to a bigger nest egg. Here’s a brief list of considerations:

• How are rebates calculated? (Do certain categories pay more than others?)
• Are there limits to what you can earn? (How would you do in an average year?)
• What’s the interest rate? (How much will it cost if you slip up?)

The best brokerage cards combine a generous cash back yield with unlimited earnings potential. Does Fidelity’s Investment Rewards card. issued by American Express (AXP ), measure up? That’s the question before us today in this ongoing series examining the best (and worst) credit card offers out there .

3 Important Details About the Fidelity Investment Rewards Card

Fidelity offers multiple cards to accountholders. There’s a Visa card on top of two other Amex options: one specifically geared for college earning, and another for retirement savings. The Investment Rewards card is the most flexible option. Here’s a closer look at the benefits and bugaboos:

1. A straight 2% back. Fidelity makes earnings simple with 2% cash back on all retail purchases. Like most retail cards, the program kicks off two points for every $1 spent. Reach 5,000 points and you become eligible to apply for a $50 cash rebate to the account of your choice.

2. Account flexibility. Fidelity allows cardholders to designate deposits automatically or choose where and when to transfer earnings. All Fidelity accounts are eligible. So if you earn, say, 10,000 points on $5,000 in quarterly spending — and you have both a Fidelity IRA and a 529 account for your child — you’re allowed to contribute all $100 to one, or split it any way you see fit. Just be sure your contributions don’t exceed state and federal limits.

3. Beware the balance transfer. Please don’t apply for this card with the intent of moving a balance from elsewhere. Yes, you do get seven billing cycles of 0% interest on a transfer, but the 4% fee you’ll pay on the transferred balance is simply too high. Nor will you earn points for handing Amex a nice new chunk of change.

Interestingly, you don’t have to use Investment Rewards to earn portfolio cash. Members may also select merchandise from the poorly rated WorldPoints catalog, or use points for booking travel. Forgo these options if you can. Rewards cards offer better travel options. while retail cards offer better deals for compulsive shoppers.

This Is the Right Card for You If.

Apply for this card if you’re already socking away cash for emergencies and have a brokerage account for saving for long-term needs such as education and retirement.

Earning interest and dividends on free cash will enhance your earnings power, and could bulk up your portfolio over time. You might even save enough to buy stock in a company with generous shareholder perks.

Regardless, it’s important to remember that Amex wouldn’t offer this card if it didn’t produce profits. A 2% kickback to your retirement account won’t mean much if you pay the card’s minimum 13.99% interest rate on a hefty balance.

What credit cards do you use? What is the best credit deal you’ve found? Please let us know using the comments box below.

Motley Fool contributor Tim Beyers didn’t own shares in any of the companies mentioned in this article at the time of publication. Check out Tim’s portfolio holdings and past columns. The Motley Fool owns shares of Bank of America. Motley Fool newsletter services have recommended buying shares of Charles Schwab and writing a covered strangle position in American Express .